1. Field of the Invention
This invention relates to an electronic attenuator for attenuating a noise by making a reversal sound act on the noise and by a mutual interference action of both.
2. Description of the Prior Art
Conventionally, it has been known to attenuate a noise actively by making a reversal sound of reverse phase and same sound pressure act on the noise and by mutual interference of the noise and the reversal sound. Especially in recent years, with the progress of the electronic device, signal conditioning techique, etc., it has been suggested, for example, to generate a reversal sound which is faithful to a noise and to attenuate even an unsteady-state noise by generating a reversal sound in immediate response to variations of noise. Japanese Patent Application Laying Open Publication Gazette No. 61-296329 (title of the invention: electronic attenuating system) is an example.
A sound attenuating device of this kind uses a speaker for generating a reversal sound, for example, a speaker is provided at a circumferential wall of a duct connecting to a noise source and reversal sounds emitted from the speaker are caused to interfere with noises in the duct. Such a speaker is generally of cone type but is required to be of a certain size so as to generate a sound pressure of almost the same extent as a noise and also is required to have fairly outstanding characteristics so as to generate reversal sounds faithful to noises.
In the case where a conventional sound attenuator is applied to the sound attenuating for a device of small scale, such as an air conditioner for use at a shop, a home or the like, it is inevitable that a speaker occupies a large space and accordingly a noise source device is larger in size or in the case where a speaker is placed at the optimum position, a large protrusion is formed at a case body. Thus, application of it to the device of small scale itself was impossible and also was difficult from the cost point of view.
In the case of attenuating a specific noise source in a duct or a case body which is smaller than a size of certain extent, it is impossible to secure space for placing a speaker and accordingly the application of a sound attenuator is limited to a large duct an a large device.